The invention relates to array antenna systems and, more particularly, to such systems employing adaptive signal processing or beamforming to address aspects of jamming or interference.
Various forms and implementations of adaptive processing of signals received by array antennas have been suggested in order to provide antenna gain reduction in the direction from which a jamming signal emanates. Thus, if a pattern null or reduced gain characteristic can automatically or adaptively be provided at the appropriate azimuth, system sensitivity to jamming signals arriving at that azimuth will be reduced.
Application of perturbation sequences to adaptive beamforming has been described. Such techniques may be appropriate to enable adaptive processing in applications in which an antenna output representing a summation of signals received is available, but it is not possible or convenient to make available each signal received via individual elements of an array antenna. See for example, the article, “Application of Orthogonal Perturbation Sequences to Adaptive Beamforming” (IEEE Transactions on Antennas and Propagation, Vol. AP-28, No. 2, March 1980, pp. 191-202).
In addition to reduction of the effects of jamming or interference signals, it may be desirable to determine the actual direction from which such signals emanate. Thus, if available, information as to the direction of a jamming or interference signal might be used as the basis for action suitable to accomplish a cessation or reduction of such signal. However, known types of adaptive processing systems have not provided a direction finding capability.
Various forms and implementations of signal processing for the purpose of location or direction finding relative to the source or emitter of what may be a jamming or interference signal have been described. Prior known approaches have been based on the availability and processing of individual signals as received by each of a plurality of antenna elements in order to construct and analyze a matrix of cross correlations between the signals received by the individual elements. As a result, for an array antenna including a significant number of receiving elements, parallel processing of each received component signal and matrix analysis for this purpose can require provision of fairly extensive signal processing hardware and computational capacity. See for example the articles, “Multiple Emitter Location and Signal Parameter Estimation” and “Multiple Source DF Signal Processing: An Experimental System” (IEEE Transactions on Antennas and Propagation, Vol. AP-34, No. 3, March 1986, pp. 276-280 and 281-290, respectively).
An example of an area of interest is provision of adaptive array anti-jam and direction finding capabilities for use with Global Positioning Satellite (GPS) system array apertures of reduced or compact size. While small array configurations may be provided for adaptive processing usage, there are constraints on size, weight, cost and thus computational capacity in missile guidance and other applications.
As background, in existing adaptive processing applications, the most commonly used analog adaptive weight control is based on using the Least Mean Square (LMS) algorithm. The performance metric is the minimization of the output power from an array, subject to a weight vector constraint to avoid system shut down. This constraint can be addressed by use of a fixed weight on a reference input mode whose spatial response is high over the entire region from which desired satellite signals may emanate. A typical configuration, as diagramed in FIGS. 1 and 2 is based on taking small steps in the opposite direction of the instantaneous gradient of the output power with respect to the weight vector components. The instantaneous estimate of the gradient vector components are formed by multiplying the current value of the array outputs by the conjugate of the current value of each input. Since a received GPS satellite signal level can be expected to be sufficiently below the level of the undesired background or jamming, the received signals will not impact the adaptation process.
Although generally considered lowest in cost, the LMS algorithm can suffer from sub-optimal convergence properties in some scenarios. In addition, since this approach directly estimates the gradient, it does not provide a viable method for estimating the correlation matrix of the interference background, which could be used for direction finding. Without data providing an estimate of the correlation matrix, existing subspace direction finding algorithms, such as the MUSIC algorithm described in articles identified above, can not be applied for source location purposes.
For many applications, military or other, some or all of space, size, cost, power and data transmission constraints, as well as co-location and cabling constraints, may foreclose or make impractical the use of known signal source location or direction finding techniques, such as those referred to above. As a result, even though there may be provided an array antenna system including adaptive processing for reduction of jamming or interference effects, there may be no applicable location or direction finding arrangements capable of being implemented on a cost-effective and size and form basis which meets the applicable constraints.
Objects of the present invention are, therefore, to provide new and improved systems and methods applicable to enabling signal source location and which may overcome one or more of cost, size and other constraints applicable to implementation of previously available arrangements.